Signaling system



Oct. 2g, 1946. R. A. CLARK, JR

SIGNALING-- SYSTEM 2 Sheets-Sheet l Filed. Feb. 17, 1944 ROBERT A. CLARK JR.

ATTORNEY Oct, 29: 1946. R. A. CLARK, JR

SIGNALING- SYSTEM Filed Feb. 17,' 1944 2 Sheets-Sheet 2 NGE,

INVENTOR. ROBERT A. CLARK JR.

ATTORNEY Patented Oct. 29, 1946 UNITED STATES PATENT OFFICE SIGNALING SYSTEM Robert A. Clark, Jr., Chicago, Ill.

Application February 17, 1944, Serial'No. y522,742

7 Claims.

The present invention relates to signaling systems and more particularly to improved apparatus for discriminating between two different signals which may be non-concurrently transmitted over a signal channel of a telephone or similar signaling system.

In conventional telephone systems it is usually necessary that each voice channel be utilized not only for voice signal transmission, but also for the transmission of supervisory signals, such for example as ringing signals, flashingrecalls signals and the like. Further, in long lines networks, utilizing repeaters for voice current gain, it is highly desirable to use alternating supervisory signals having a xed frequency within the voice frequency rangezto provide the desired signal indications. This has previously been accomplished by modulating a voice frequency carrier at a low frequency, and utilizing the low frequency component lof the signal for supervisory signaling purposes. It has been found, however, that with such arrangements, voice signals traversing the signals channel may produce a false response of the supervisory signal detecting equipment, if the level of voice signal transmission is sufficiently high.

It is an object of the present invention, therefore, to provide improved and exceedingly reliable methods and apparatus for` discriminating between voice and supervisory signals which may be transmitted over a voice signal channel.

More generally stated, it is an object of the present invention to provide improved methods and apparatus for discrimination between a rst signal having different frequencies Within a band and a second signal having a fixed frequency within the band when the signals of different types are non-concurrently received overl the saine signal channel.

According to a further object of the invention, those components of a received first or voice signal which have frequencies other than the fixed frequency of the second orl supervisory signal are utilized lto` prevent the'apparatus for detecting the second or supervisory signal from responding to a component of a received first signal which has the same frequency as the second signal.

In accordance with a further and more specific object of the invention, a four terminal bridge circuit, which is balanced only at the fixed frequency of the second yor supervisory signal, is utilized invdiscriminating between the signals of different characters.

It is still another object of the invention to provide an improved circuit network which is simple in arrangement and is positive and reli-v in which Figs. 1 and 2, when laid end to end in the order named, illustrate a portion of a telephone system having embodied therein a signal discriminating network characterized by the features of the present invention.

Referring now to the drawings, the equipment there illustrated may be provided in the toll exchange of a metropolitan central exchange area or alternatively may be located in an isolated private branch exchange which is connected with a central exchange area by means of trunks requiring voice current amplification for satisfactory communication thereover. In brief, this equipment comprises an operator switchboard III having a jack Illa, a ringing signal in the form of a drop I 0c, and a pad switching key Id associated with a signal channel to a distant oilice. This channel is divided within the exchange into an incoming signal channel II and an outgoing signal channel I2 in order to permit suitable filter and .amplifier facilities to be provided for insuring high level, high fidelity transmission between the switchboard I 0 and the distant oflice. If carrier transmission is utilized between the illustrated exchange equipment and the distant oice, the bracketed terminals of the two signal channels II and I2 will of course extend to conventional composite equipment through which they are coupled to a specified carrier channel. On the other hand, if a two conductor trunk is utilized to connect the illustrated exchange equipment with the distant office, the identied channels will of course be coupled to the trunk through a conventional hybrid system. Within the exchange, the two channels I I -and I2 are coupled to a two-way link extending tothe call and answer jack Illa by means oiga conventional hybrid system I4 and pad or transmission loss network I3, both of which are entirely conventional in arrangement.` The network I4 is, of course, utilized to determine the level of voice current transmission over a connection set up by way of the illustrated equipment. The impedance of the network I3, as combined with the equivalent impedance of an operator cord circuit connected with the tip and ring contacts of the jack lila, is balanced by means of a balancing network I5, thereby to effectively isolate the two channels l! and I2 and thus minimize the transmission of signals from the channel II to the channel I2 through the hybrid system I4. Switching relays 60 and Til arrange for control by the pad key Illd are respectively provided for switching in` and out signal loss resistors within the network I3 and correspondingly changing the impedance of the balancing network 'Ill to maintain the hybrid system I4 in a balanced condition. A iilter and amplifier. network 9, which may be entirely conventional in character, is provided in the outgoing signal channel I2 for the purpose of raising the level of voice transmission to the desired value. Similarly, the incoming signal channel II includes an amplifier and filter network I6 and a iinal audio amplifier stage lfI, arranged in tandem in the order named. The final amplifier stage Il includes an electron discharge tube I8 provided with an output circuit which is directly coupled to the input terminals of the hybrid system Id.

For the purpose of discriminating between voice signals and supervisory signals traversing the incoming signal channel II, a bridge circuit indicated generally at I9 is coupled to this channel through an adjustable potentiometer resistor 22T whichshunts `the channel at the output side of the tube I3 and is paralleled by a lter condenser 23; More specically, the bridge circuit IQ'incl'udes a voltage step up transformer 2l` having a primary winding 2id bridged across an adjustable portion of the potentiometer resistor 22 by means of the wiper 22a which is adustable along this resistor. The transformer 2l also includes a secondary winding having a tapped midpoint I'Sa, such that equal voltages are developed in the two sections 2lb and 2I'c thereof when the primary wind 2 Ia is energized' at any frequency within the audio frequency range. The two identified sections ofthe transformer secondary winding respectively comprise two legsA of I the described'. bridge circuit. A third leg of the' bridge circuit is made up of a parallel resonant circuit 2'4 which comprises the shunt connected inductance and capacitance elementsY 24u and 2`4b. This resonant circuit is tuned exactly to the fixed frequency within the audio frequency range at which supervisory signals are transmitted to the input side of the amplifier and filter network I6. Thus if a xed frequency of 150'0 cycles is to be utilized as the supervisory signaling frequency, the circuit 24 is tuned exactly to this frequency. For the purpose of matching the terminal impedance of the circuit 24v at this frequency, the fourth leg of the bridge circuit is made up of an adjustable resistor 25 having a wiper 25a movable therealong for the purpose of varying the encircuited portion of the resistor. It will thus be apparent that the bridge circuit I9 comprises four legs` 2Ib, 2Ic, 24 and 25 having four terminals. lea, IQb, ISc and Idd, and that a voltage appearing upon theincoming signal channel Il is equally'impressed.. upon the two legs 2lby and llo thereof through the inductive coupling between these legs and the primary winding 2Ia. It will also be understood that at resonance the impedance of the tuned circuit 24is substantially resistive and hence may be entirely balanced by suitable adjustment of the wiper 25a along the resistor 25.

In order to detect a supervisory signal appearing upon the incoming signal channel I I and impressed upon the bridge circuit I9 in the manner just explained, detecting means are provided which comprise a three electrode thermionic tube 213 and a signal repeating relay 4) shunted by a delay release condenser dilo. More specifically, the output circuit of the tube 20 includes the winding of the relay li and the upper part of a voltage dividing resistor 29. The input electrodes of the detecting tube 2i) are bridged across the resonant circuit 2d in series with a decoupling resistor 26a, a biasing resistor 2l and a source of biasing potential represented by the voltage appearing across the section 29a of the voltage dividingvresistor 23. With this arrangement and as will appear later,'when the bridge circuit I9 is energizedat the particular frequency of 1500 cycles at which it is balanced, the voltage drop across the resonant circuit 29 is detected by the tube 2li and utilized to effect operation of the signal repeating relay @di For the purpose of utilizing a' received voice signal to develop a bias voltage which may be applied to the tube 2) to prevent the relay di) from operating, a biasing network is provided which is coupled to the terminals IBa and ld of the bridge circuit, and is utilized to develop a bias voltage across the biasing resistor 2l. More specifically, this network comprises a transformer 26 having a primary winding 26a bridged between the terminals ISa and ld, and a secondary winding 2Gb which is bridged across the biasing resistor 2l in series with the space current path of a diode rectifier 3U. The resistor 2l is shunted by a smoothing condenser 28 which is utilized to maintain a direct voltage across the resistor 21 when the primary winding 25a of the transformer is energized.

In' the illustrated arrangement of the equipment, incoming supervisory signals are utilized only for the purpose of controlling the drop Ic associated with the jack I lia. To this end, the relay Il@ is arranged directly to control a ringing relay 5t having the function of cutting off the padV network I3 from the jack Illa, opening the incoming signal channel Il, and connecting the tip and ring contacts of the jack Ilia to the source of ringing current which is provided in the exchange. In this regard it will be understood that the exchange in which the illustrated equipment is located includes a conventiona1 exchange battery having a grounded positive terminal and a negative terminal connected to each of the illustrated relay terminals identified by the negative polarity sign.

vBrieiiy to consider the operation of the above described equipment, it will be understood that when this equipment is seized in the distant oice on a call to the operator switchboard II), a ringing signal of the fixed frequency selected for supervisory signaling and originating at the distant end of the connection is impressed upon the input side of the signal channel II. More specically, this signal has a frequency exactly equal the particular frequency to which the resonant circuit 24 istuned. It is amplified through the network I6 and the final amplifier stage I'I and appears across the potentiometer resistor 22. That portion thereof which appears across the lower part of the resistor 22 serves to energize the winding 21a', thereby to produce equal voltages across the two legs 2'Ib and 2Ic of the bridge circuit i9. Since at the particular frequency indicated, the resistive impedance of the resonant circuit 2li is matched by the resistance of the potentiometer resistor 25, substantially-no voltage appears b etween the bridge circuit terminal I'Ba' andy lad, so

that the Voltage input to .the biasing network. comprising ithe" diode. rectifier 3U is negligible. The alternating voltage yappearing across the res-f' the bias voltage appearing across vthe resistor secftion 29a, so that the current iiow through this.

tube and the winding of the signal repeating relay 40 is substantially increased'. As a result, the relay 40 is caused to operate. relay opens its contacts 42 to interrupt the path normally short circuiting the winding of the ringing relay 3B, and closes its contacts 4Ito energize the relay 5i! from the exchange battery. The relay 50 upon operating. opens its contacts 5I to interrupt the connection between the hybrid system I4 and the linal ampliiier stage I'I of the incoming signal channel II. At its contacts 52 and 54, the relay opens the connection between the jack Ilia and the pad network I3. At its contacts 53 and 55, the relay 50 completes a circuit for energizing the winding of the drop IUc from the ringing current source. When thus energized, the drop Illc is actuated to provide an indication that an incoming call has been routed to the jack Illu which should'be answered.

Upon termination of the ringing signal appearing at the input side of thesignal channel I I, the

bridge circuit I9 is obviously de-energized, per-l mitting the bias voltage across the resistor section 29a to so reduce the Vspace current iiow through the detector tube 20 as to eilect the re-v lease of the signal relay 40. Upon restoring, this relay opens its contacts 4I to deenergize lthe relay 5D and closes its contacts 42 to' recomplete the path for short-circuiting the winding of the relay 50, thereby to render the latter relay slow to release. After a short interval, the relay 53 In operating, thisV restores and opens its contacts 53 and 55 to dev energize the winding of the drop IEic. At'its contacts 5I, the relay 50 recompletes the connection between the hybrid system I4 and the final amplifier stage I1'. At its contacts 52 and 54 the relay 5i] recompletes the connection between the jack Ilia and the pad network I3. Thus the illustrated equipment is conditioned for two-way voice communication over a connection set up by way of the jack Illa.

In order to extend the call from the jack Illa to the desired point, the operator attending lthe switchboard I0 or that portion thereof at which the identified jack is provided, may insert the answer plug of her cord circuit in the jack Illa to establish a connection between her cord circuit and the calling operator. Incident to this operation, the jack contacts Ib and Ille are opened to disconnect the winding of the drop Ille from the two sides of the signal channel. Voice signals originating at the distant end of the connection and appearing at the input side of the signal channel II are amplified through the net-V work I6 and the final amplifier stage I'I, andare transmitted through the hybrid system I4 and the pad network I3 to the receiver of the operators head set for reproduction. In this regard it will be understood that the hybrid system I4 acts to prevent signal energy transfer from the channel II to the channel l2, thereby to eliminate any possibility of singing. Voice currentsl originated by the operator handling the call at the switchboard ID, are similarly transmitted through the pad network I3, the hybrid system I4 and the' amplifier network 9 for transmission- Thus a two-way comf to the distant operator. munication channel is set up between the calling and answering operators over which instructions may be transmitted to the answering operator for the extension of the call. The manner in which the call may be further extended to the desired destination is exactly the same as that followed in conventional manual oiiice or toll ofce practice. In this regard it is pointed out that depending upon the destination of the call, a variable amount of voice current gain may be required for satisfactory communication between the calling and called parties. Thus if the call is rto be extended over a long high loss line, considerable voice current gain may be required in the illustrated equipment to insure an adequate level of signal reproduction at the calling and called ends of the connection. In such case the pad key IIld may be operated for the purpose of energizing the two switching relays 60 and 1I), thereby to cut out thetransmission loss pads in the network I3 and to correspondingly change the impedance of the network I5 to provide for a continued balance of the-hybrid system I4. If, on the other hand, the call is'to be extended over a short line having good voice current transmission characteristics such that only the normal Vamount of signal gain through the illustrated equipment is required, the loss pads of the network I3 are retained in the circuit.

In this regard it will be understood that voice signals originating at the distant end of the connection and appearing at the input side of the signal channel II may comprise different composites of all frequencies within the audio frequency range. The received voice signals may or may not include a component having the particular frequency to which the resonant circuit 24 is tuned. Moreover, any received voice signal having the particular frequency t0 which the resonant circuit 24 is tuned will always be accompanied by other components having frequencies differing from this particular frequency. Also, at

- all frequencies withinv the voice frequency range other than the particular frequency to which the circuit 24 is tuned, the terminal impedance of the resonant circuit 24 is -diierent from that of the resistor 25. Thus, the bridge circuit I9 is unbalanced at such frequencies, so that a Voltage is developed between the terminals I9a and I9d thereof. It will be understood, therefore, that so long as vvoice 'current transmission over the channel I I continues, a substantial voltage is impressed upon the primary winding 26a of the transformer 2B. This voltage appears in increased magnitude across the secondary winding 2Gb and is rectified by the diode rectifier 30 to appear as a direct current biasing voltage across the biasing resistor '21. Moreover, the direction of current flow through the diode 30 is such that the Voltage developed across the resistor 21 is additive with respect to the voltage across the resistor section 29a, whereby the total Ibias voltage impressed between the input electrodes of the tube 2B through ythe inductance element 24a is substantially increased. This increased bias voltage as applied to the detector tube 2li opposes the voltage appearing across the resonant circuit 24, thereby to prevent .the relay 40 from operating even though a voice signal component of the particular frequency to which the circuit 24 is tuned appears yat the input side of the signal channel I I. Thus ,Y the relay 40 is positivelyr prevented agrar/ref fromfoperating. when thechannel Il. is being utilized for voice signal transmission. l

After ahconnection hasy been set up between twosubstations through the illustrated equipment in the. manner explained above, it maybe released under the control of the calling operator andthe operator handling the call at the switchlboard iii in an entirelyr conventional manner. Further, the manner in which calls may be routed 'from the switchboard le through the-illustrated equipment to the distant cnice is entirely conventional and will be readily apparent to those skilled in the art.. In this regard it is pointed out that in the interests of simplifying the disclosure, the equipment for transmitting supervisory signals from the switchboard lll through the illustrated circuit equipment to the distant office' has not Ibeen illustrated in the drawings.

From the above explanation it will be understood that in order to obtain an exact balancing of the circuit iii. atl the particular frequency to which the resonant circuit 24 is tuned, the impedance across the terminals of the circuit 24 at this frequency must balance the impedance of the resistor 25. The tuned circuit 24 is, however, shunted by the grid-cathodel impedance ofthe tube 20, which impedance is variable with the grid voltage, and if not decoupled from the tuned circuit tends to prevent the proper bridge circuit balance from being obtained. By providing the series resistor a, however, havin'ga resistance substantially greater than'the grid-cathode impedance of the tube 20, the latter impedance is rendered substantially non-effective to upset the balance of the circuit I9 at the particular frequency to which' theV resonant circuit is tuned, regardless of the voltage input to the bridge circuit. tions of resonance of thecircuit 24, theV voltage appearing across the secondary winding 26h is less than five percent of the voltage across the tuned circuit 24. When, on the other hand, the bridge circuit is excited at any frequency-substantially removed from the resonant frequency of the tuned circuit 24, the voltage across the winding 2thmay exceed' by ten times the voltagev across the tuned circuit 24. These relative values indicate'the extreme sensitivity of the network to frequency deviations from the particular value to which the circuit 2li' is tuned.

Although the improved signal discriminating network has been disclosed in its operation to discriminate between voice and ringing signals appearing upon the same channel of a telephone circuit, it will be understood that this network is adapted for use in any signaling system where fixed frequency and multi-frequency signals are to be received over the same signal channel, but only the fixed frequency signal is to be detected. llt` will also be understood that regardless of the particular system in which it is used, the action of the network is such that a false response of the detecting equipment during transmission of the multi-frequency signal is postively prevented.

While one embodiment of the invention has been disclosed, it will be understood that various modification may be made therein, which are within the true spirit and scope of the invention.

Il claimt 1. In a signaling system whichincludes a signal channel over which may be non-concurrently transmitted a first signal having different frequencies within a frequency band or a second signal having a fixed frequency within said band, a four terminal bridge circuit coupled to 'said' It has been determined that under condi-V channel and balanced. at said .fixed frequency,

said circuit including a parallel resonant circuit.

in one leg thereof, a biasing resistor, thermionic means, including inputv electrodes bridging said resonant circuit in series with said resistor to detect a second' signalappearing on said channel, and means coupled to two opposed terminals of said bridge circuit and responsive to the Volttage developed acrossA said two terminals when said bridge circuit is unbalanced for developing a bias voltage across said resistor which serves to bias said thermionic' means against signal detection when a first signal appears on said channel.

2; In a signaling system Iwhich includes a signal channel over which may be transmitted a first signal having different frequencies within a frequency band or a second signal having a `fixed frequency within said band, a four terminal bridge circuit coupled to said channel, a resonant circuit tuned to said fixed frequency and included in one leg of said bridge circuit, an ad'- justable impedance included in a second leg of said bridge circuit to balance the impedance of said resonant circuit at said fixed frequency, whereby the v-oltage developed across two opposed terminals-of said bridge circuit is substantially negligible when a'second signal appears on said signal channel and becomes appreciable when a first signal appears on said channel, a biasing resistor, thermionic means including input electrodes bridging said resonant circuit in series with said resistor and operative to detect a second Isignal appearing on said channel, means coupled to said two terminals of said bridge circuit and responsive to the voltage developed thereacross for biasing said thermionic means against signal detection, thereby to prevent said thermionic means'from detecting a component of a transmitted rst signal having said fixed frequency, and means for preventing the input electrode impedance of said thermionic means from disturbing the balance of said bridge circuit at said fixed frequency.

3. In a network for discriminating between a receivedY first signal having different frequencies within a frequency band and a received second signal having a fixed frequency within said band, a four terminal bridge circuit adapted to be excited by said signals and balanced at said fixed frequency, a resonant circuit tuned to said fixed frequency and included in one leg of said bridge circuit, an impedance element included in a second leg of said bridge to balance the impedance of said resonant circuit' at said fixed frequency, whereby the voltage developed across two opposed terminals of said bridge circuit is substantially negligible when said network is excited by said second signal and becomes appreciable when said network is excited by a first signal, a thermionic tube provided with a grid and cathode bridged across said resonant circuit, means for preventing the grid-cathode impedance of said tube from disturbing the balance of bridge circuit at said xed` frequency, means in cluding said tube for detecting a second ysignal impressed on said network, and means coupled to said two terminals of said bridge circuit and responsive to the voltage developed thereacross for biasing said tube against signal` detection, thereby to prevent, said detecting means from detecting a component of a first signal having said fixed frequency.

4. In a signaling system which includes a signal channel over which maybe non-concurrently resonant circuit in series with said resistor to detect a second signal appearing on said channel, means coupled to two opposed terminals of said bridge circuit and responsive to the voltage developed across said two terminals when said bridge circuit is unbalanced for developing a bias voltage across said resistor which serves to bias said thermionic means against signal detection when a first signal appears on said channel, and means for preventing the input electrode impedance of said thermionic means from disturbing the balance of said bridge circuit at said fixed frequency. v

5. In a signaling system which includes a signal channel over which may be non-concurrently transmitted a rst signal having diiferent frequencies within a frequency band or a second signal having a fixed frequency Within said band, a four terminal bridge circuit coupled to said channel and balanced at said vfixed frequency, said circuit including a parallel resonant circuit in one leg thereof, a biasing resistor, thermionic means including input electrodes bridging said resonant circuit in series with said resistor to detect a second signal appearing on said channel, a transformer provided with a primary winding bridged across two opposed terminals of said bridge circuit and including a secondary Winding having a voltage developed thereacross in response to excitation of said bridge circuit at frequencies other than said xed frequency,v

and means including a thermionic rectier connected in series With said secondary winding and said resistor for converting said last-named voltage into a bias voltage across said resistor which serves to bias said thermionic means against detection of a first signal appearing on said chanquencies within a frequency band or a second signal having a fixed frequency within said band, a four terminal bridge circuit coupled to said channel and balanced at said xed frequency, said circuit including a parallel resonant circuit in one leg thereof, a biasing resistor, thermionic means including input electrodes bridging said resonant circuit in series with said resistor to detect a second signal appearing on said channel, a transformer provided with a primary winding bridged across two opposed termina-ls of said bridge circuit and including a secondary winding having a voltage developed thereacross in response to excitation of said bridge circuit at frequencies otherk than said xed frequency, means including a thermionic rectifier connected in series with said secondary winding and said resistor for converting said last-named voltage into a bias voltage across said resistor which serves to bias said thermionic means against detection of a rst signal appearing on said channel, and means for preventing the input electrode impedance of said thermionic means from disturbing the balance of' said bridge circuit at said Xed frequency.

7. In a network for discriminating between a received iirst signal having different frequencies within a frequency band and a received second signal having a fixed frequency within Said frequency band, a four terminal bridge circuit adapted to be excited by said signals and balanced at said nxed frequency, means included in said circuit for unbalancing said circuit at frequencies other than said fixed frequency within said frequency band, thermionic means including input electrodes coupled to one leg of said bridge circuit, said thermionic means being operative to detect excitation of said bridge circuit at said fixed frequency, means coupled to two opposed terminals of said bridge circuit and responsive to the voltage developed across said two terminals when said bridge is unbalanced for biasing said thermionic means against signal detection, and means for preventing the input electrode impedanceof said thermionic means from disturbing the balance of said bridge circuit at said xed frequency.

ROBERT A. CLARK, JR. 

